Synthesis of process gas by direct cooling with nitrogen

Abstract

Synthesis method of a process gas rich in H2 / CO, or in NOx, or in HCN, or in CH2O, for the subsequent production of at least a chemical among hydrogen, ammonia, methanol, olefins / paraffins / alcohols by Fischer Tropsch synthesis, nitric acid, hydrogen cyanide, formaldehyde, comprising a direct cooling with nitrogen. The process gas is discharged at high temperature from a steam and / or CO2 reforming reactor or from an adiabatic oxidation reactor with oxygen-containing gas. The process gas at high temperature, before next unitary operations, is partially or totally cooled by mixing with a colder nitrogen stream.

Description

SYNTHESIS OF PROCESS GAS BY DIRECT COOLING WITH NITROGENField of the inventionThe present invention is a method for synthesizing process gases comprising a direct cooling with nitrogen.Specifically, the present invention relates to process gases discharged at high temperature from a chemical reactor where it is performed the steam or carbon dioxide reforming process, or the adiabatic oxidation process with gas containing oxygen, of hydrocarbons and / or ammonia and / or methanol, for the subsequent production of chemicals such as hydrogen (H2), ammonia (NH3), methanol (CH3OH), Fischer Tropsch synthesis products, nitric acid (HNO3), hydrogen cyanide (HCN) or formaldehyde (CH2O).The process gases object of the present invention are normally synthesized and discharged from the relative reforming / oxidation reactors at operating temperatures higher than 500-550°C, often higher than 700-750°C, and at operating pressures higher than 0.12MPa(a), often above 0.20MPa(a), often above 0.45MPa(a), and often above l.OOMPa(a).Process gases discharged at high temperature from reforming / oxidation reactors always require a cooling operation before undergoing further unitary operations; consequently, the synthesis methods of aforementioned process gases comprise a cooling operation downstream of the reforming / oxidation reactor. Sometimes, the cooling must be rapid (“quenching”) to stop residual chemical reactions.The cooling operation of the aforementioned process gases is conventionally carried out by indirect heat exchange in process boilers and / or heat exchangers installed downstream of the reforming / oxidation chemical reactors. Consequently, the process gas is cooled by high pressure steam generation, steam superheating or boiler feedwater preheating. Alternatively, high temperature process gases containing solid carbonaceous or waxy substances are sometimes cooled by direct heat exchange with water, steam or hydrocarbons (quenching).Purpose of the inventionCooling of process gases discharged from above-mentioned reforming / oxidation reactors by conventional indirect heat exchange is a critical operation because processboilers and heat exchangers often operate under severe thermo-hydraulic and thermomechanical conditions and because the process gas is often chemically aggressive relative to steels. Furthermore, the adiabatic oxidation of hydrocarbons and / or ammonia and / or methanol also involves risks of temperature peaks of the process gas and therefore risks of overheating or damage to the equipment downstream of the oxidation reactor.Cooling of process gases by conventional direct heat exchange is also a critical operation because the cooling fluid mixed with the hot gas can undergo chemical reactions or change the chemistry of the synthesis, or because it involves a high consumption of water.The process gas synthesis method with direct cooling with nitrogen, object of the present invention, represents an alternative and efficient technical solution in improving safety and productivity with respect to synthesis methods based on conventional cooling.The synthesis method described herein comprises a direct heat exchange operation between the high temperature process gas discharged from the reforming / oxidation reactor and a colder nitrogen stream; in other words, the synthesis method described here comprises a mixing operation between the hot process gas to be cooled and a cold nitrogen stream.It is emphasized that, according to the present invention, the direct cooling operation with nitrogen is carried out on the process gas discharged from the reforming / oxidation reactor; therefore, in other words, the direct cooling operation with nitrogen is realized directly downstream of the reforming / oxidation operation and upstream of any other unitary operation, as a chemical synthesis, a distillation or a washing.Moreover, it is emphasized that the direct cooling operation with nitrogen related to the present invention does not foresee any stream or chemical species removal from the process gas following the cooling; consequently, the direct cooling operation with nitrogen is an operation clearly distinct from operations apt to separate chemical species, as a washing or purification operation.The nitrogen, since chemically inert, does not attack steels and does not substantially modify the chemistry of the synthesis; furthermore, the cooling nitrogen acts as a chemical reagent if the process gas is used for the subsequent production of ammonia.However, it should be noted that mixing with nitrogen increases the volume of the process gas and reduces the partial pressure of the reacting chemical species.The advantages of the synthesis method object of the present invention are:- The use of a chemically inert cooling fluid and therefore not subject to consumption, not altering the chemical synthesis and not attacking the steels;- The fast cooling of the process gas;- The limited pressure drop related to the mixing;- The possibility of controlling the synthesis of the process gas during transients by regulating the nitrogen flow;- The possibility to smooth temperature peaks caused by malfunctions or transients of the oxidation reactor;- The possibility to mitigate thermo-hydraulic and thermo-mechanical conditions of the process boilers or heat exchangers downstream of the chemical reactor;- The possibility to decrease the steam production in the process boiler or decrease the thermal load on heat exchangers;- The reduction of the chemical aggressiveness of the process gas;- The possibility to compensate a performance reduction of process boiler or heat exchangers.BackgroundMixing of cold gaseous reactants and hot gaseous reaction products is a technique sometimes used to control the temperature and yield of chemical reactions within a chemical reactor. When exothermic chemical reactions are conducted with the aid of a solid catalyst divided into catalyst beds, dilutions and intermediate cooling operations are sometimes performed by injecting cold reactants between the catalyst beds. The patent documents No. EP0026057 and No. EP0550525 describe synthesis processes and chemical reactors with catalytic beds with dilutions and intermediate cooling by injection of cold chemical reactants.High temperature process gases with a significant content of soot, dust, carbonaceous or waxy substances are often subjected to direct cooling. Patent documents No. US5431703 and No. US4054424 describe two methods for rapidly cooling a process gas stream by mixing with steam, or hydrocarbons, or mixtures of chemical reactants and inert gases.Methods and devices for rapid cooling of a hot process stream by mixing with a cold fluid are disclosed in various patent documents, for example in doc. No. US3663645 andNo. US5324486. These methods and devices specifically relate to process gases from hydrocarbons cracking, where the cold fluid for direct cooling is steam or a hydrocarbon.Finally, the patent document No. EP0618282 teaches a method for mixing and attempering a hot chemical reactant stream by recycling cooled reaction products in a process boiler.Examples of prior art documents which describe synthesis methods of the process gases object of the present invention are reported here below.Documents No. EP2723676B1, No. US2019 / 382277 Al, No. US2021 / 198104A1, No. US7470514B2 and No. US2007 / 299144A1 describe synthesis methods of process gases rich in FB / CO, for the subsequent production of ammonia and / or methanol, discharged at high temperature from reforming or partial oxidation reactors. Based on these synthesis methods, the plants are equipped with cryogenic air separation units (ASU) to obtain oxygen and nitrogen.The doc. No. EP2723676B1, in paragraphs

[0027] -

[0029] ,

[0033] and

[0035] -

[0037] , and in figures 1 and 2, describes that the high temperature process gas is discharged from the adiabatic oxidation reactor (CPO), is cooled by a process gas boiler (PGB) or direct cooling, undergoes water gas conversion (WGS) and purification (PSA CO2 removal), and then is mixed with nitrogen coming from the air separation unit (ASU).The doc. No. US2019 / 382277A1 describes, in paragraphs

[0065] and

[0074] -

[0098] , in figure 1 and in tables 1 and 2, that nitrogen from the air separation unit (ASU) is injected into the washing unit (NWU), which is located downstream of water gas shift reactor (SWIFT) and CO2 removal.The doc. No. US2021 / 198104A1 discloses in the first claim that the process gas is mixed with nitrogen downstream of the water gas shift reactor (SHIFT). This is in accordance with the description provided in paragraphs

[0010] -

[0020] ,

[0025] and

[0026] ,The doc. No. US7470514B2 describes an operating method (column 2, line 34 - column 3, line 63) where nitrogen is injected into the process gas downstream of the water gas shift reactor (unit No. 10).Finally, also the doc. No. US2007 / 299144A1 discloses in paragraphs

[0014] and

[0024] —

[0037] that nitrogen is injected into the process gas downstream of the water gas shift reactor (reactor B).Nowhere in the cited patent documents No. EP2723676B1, No. US2019 / 382277A1, No. US2021 / 198104A1, No. US7470514B2 and No. US2007 / 299144A1 it is described that a nitrogen stream is mixed with the high temperature process gas directly downstream of the reforming / oxidation reactor and upstream of the next unitary operation, specifically of the water gas shift. Based on these cited prior-art methods, cryogenic nitrogen is used downstream of the water gas shift and carbon dioxide removal units, where the process gas is already relatively cold and lean in carbon monoxide; the nitrogen downstream of the water gas shift and carbon dioxide removal units is substantially used for washing or purify the process gas. Consequently, these prior-art methods do not teach a direct nitrogen cooling operation directly downstream of the reforming / oxidation and upstream of the next unitary operation, where the process gas has a high temperature and is rich in carbon monoxide.The doc. No. GB837030 discloses a high-temperature synthesis method of H2 / CO- rich process gas by a hydrocarbons adiabatic oxidation reactor and by direct heat exchange cooling with water performed downstream of the oxidation reactor and upstream of the water gas shift reactor.The doc. No. GB1306581 discloses a method of high temperature synthesis of a process gas rich in nitrogen oxides (NOX) by an ammonia adiabatic oxidation reactor and by indirect heat exchange cooling in a process boiler and heat exchangers installed downstream of the oxidation reactor and upstream of the washing and absorption columns.The doc. WO2015006548 discloses a high temperature synthesis method of a process gas rich in hydrogen cyanide (HCN) by means of an adiabatic oxidation reactor of an ammonia / methane mixture and by rapid cooling by indirect heat exchange in a first process boiler and then in a second process boiler installed downstream of the oxidation reactor and upstream of the absorption column.The doc. No. US4450301 discloses a high temperature synthesis method of a process gas rich in formaldehyde (CH2O) by means of an adiabatic partial oxidation reactor ofmethanol and by indirect heat exchange cooling in a process boiler or heat exchanger installed downstream of the oxidation reactor.The available literature, in general, does not teach a method for the synthesis of process gas, discharged at high temperature from a chemical reactor, comprising a direct nitrogen cooling performed directly downstream of the reactor.More precisely, the literature does not describe methods for synthesizing process gases discharged at high temperature from reforming / oxidation reactors rich in H2 / CO for the subsequent production of ammonia, or rich in NOXfor the subsequent production of nitric acid, or rich in HCN, or rich in CH2O, comprising the direct cooling of the process gas by mixing with a colder nitrogen stream performed just downstream of the reforming or oxidation operation.Brief description of the inventionAccording to the synthesis method described here, the process gas discharged at high temperature from the reforming / oxidation chemical reactor is subjected to cooling by direct heat exchange with nitrogen, i.e. the hot process gas is mixed with a colder nitrogen stream; the mixing is realized directly downstream of the reforming / oxidation operation and, therefore, upstream of any other unitary operation such as the water gas shift reaction, the carbon dioxide removal, the washing or purification or concentration of the process gas.The stream for cooling the process gas is constituted of nitrogen or essentially constituted of nitrogen.The synthesis method object of the present invention refers to the following process gases:- gases rich in hydrogen (H2) and carbon monoxide (CO) synthesized in steam or CO2 hydrocarbons reforming reactors or in hydrocarbons adiabatic oxidation reactors with oxygen-containing gas for the subsequent production of hydrogen (H2), ammonia (NH3), methanol (CH3OH) or olefins / paraffins / alcohols by Fischer Tropsch synthesis;- gases rich in nitrogen oxides (NOX) synthesized in ammonia adiabatic oxidation reactors with oxygen-containing gas, for the subsequent production of nitric acid (HNO3);- gases rich in hydrogen cyanide (HCN) synthesized in ammonia / methane mixture adiabatic oxidation reactors with oxygen-containing gas, for the subsequent production of hydrogen cyanide;- gases rich in formaldehyde (CH2O) synthesized in methanol adiabatic partial oxidation reactors with oxygen-containing gas, for the subsequent production of formaldehyde.Process gases rich in H2 and CO synthesized in steam and / or CO2 hydrocarbons reforming reactors, or in hydrocarbons adiabatic oxidation reactors with oxygencontaining gas, are usually discharged at operating temperatures above 650-700°C, often above 800°C, and often above 950°C, and at operating pressures above l,0MPa(a), often above 2,5MPa(a). Steam and / or CO2 reforming reactors are operated in the presence of solid catalysts (reforming reactors of flame / radiative type or convective heat exchange type); adiabatic oxidation reactors can be operated with solid catalysts (autothermal reactors, secondary reforming reactors or catalytic partial oxidation reactors) or without solid catalysts (partial oxidation or gasification reactors). Steam reforming proceeds according to the general reaction (considering methane as hydrocarbon) CH4+H2O^3H2+CO whereby methane and water form hydrogen and carbon monoxide. CO2 reforming proceeds according to the general reaction (considering methane as hydrocarbon) CH4+CO2^2H2+2CO whereby methane and carbon dioxide form hydrogen and carbon monoxide. Adiabatic oxidation with oxygen-containing gas proceeds according to the general partial oxidation reaction (considering methane as hydrocarbon) CH4+1 / 2O2->2H2+CO whereby methane and oxygen form hydrogen and carbon monoxide. The H2 / CO rich process gas discharged at high temperature typically has a molar concentration of H2 greater than 20%, often greater than 30%, and often greater than 40%, and typically a molar concentration of CO greater than 3%, often greater than 5%, and often greater than 10%.It is emphasized that the general reforming reactions with steam or CO2 or the general adiabatic oxidation reactions with oxygen-containing gas of the hydrocarbons, as described above, are referred to methane for simplicity. If the reforming or adiabatic oxidation is carried out with heavier hydrocarbons, the general reactions are not substantially changed: steam reforming of heavier hydrocarbons proceeds according to the general reaction CnHm+nH2O^(n+m / 2)H2+nCO and adiabatic oxidation of heavier hydrocarbons proceeds according to the general reaction CnHm+n / 2O2_>m / 2H2+nCO.Furthermore, as an expert in the field knows, both the hydrocarbons reforming with steam or CO2 and the adiabatic oxidation with oxygen-containing gas also involve the water gas shift CO+FFO^FF+CCh as a secondary reaction; in other words, the general reactions of reforming with steam or CO2 and of adiabatic oxidation described above are always accompanied by the water gas shift reaction.Finally, as one skilled in the art knows, hydrocarbons steam and CO2 reforming can be conducted simultaneously in the same reforming reactor; in other words, the reactants contain hydrocarbons, steam and CO2.NOx-rich process gases synthesized in adiabatic oxidation reactors of ammonia with oxygen-containing gas are usually discharged at operating temperatures above 700-750°C and at operating pressures above 0,12MPa(a), often above 0,25MPa (to). The reactors are operated inpresence of solid catalysts. The oxidation of ammonia proceeds according to the general reaction 4NH3+5O2_>6H2O+4NO whereby ammonia and oxygen react to form water and nitric oxide; the process gas discharged at high temperature normally has a molar concentration of NO greater than 3%, often greater than 5%, and often greater than 7%.HCN-rich process gases synthesized in adiabatic oxidation reactors of ammonia / methane mixture with oxygen-containing gas are usually discharged at operating temperatures above 750-800°C, often above 1000°C, and at operating pressures above 0,12MPa(a), often above 0,25MPa(a). The reactors are operated in presence of solid catalysts. The oxidation of the ammonia / methane mixture proceeds according to the general reaction CH4+NH3+3 / 2O2_>3H2O+HCN whereby ammonia, methane and oxygen form water and hydrogen cyanide. The ammonia conversion is usually greater than 60%; normally the molar concentration of HCN in the process gas leaving the reactor is greater than 3%, often greater than 5%.CFFO-rich process gases synthesized in adiabatic partial oxidation reactors of methanol with oxygen-containing gas are usually discharged at operating temperatures above 500-550°C and at operating pressures above 0,12MPa(a), often above 0,25MPa(a). The reactors are operated in presence of solid catalysts. The partial adiabatic oxidation of methanol proceeds according to the general reaction CH3OH+1 / 2O2_>H2O+CH2O whereby methanol and oxygen form water and formaldehyde. Methanol conversion isusually over 60%; normally, the CH2O molar concentration in the process gas leaving the reactor is greater than 3%, often greater than 5%.In this description, the term "process gas" therefore refers to a process gas, discharged at high temperature from a reforming / oxidation chemical reactor, equivalent to those described above.As one skilled in the art knows, all the above general chemical reactions give an overall representation of the reaction mechanism; the actual chemical reaction mechanism may be more complex and proceed with multiple reactions in parallel and / or in series. For example, the general reaction of partial oxidation of methane to form a gas rich in H2 / CO (CH4+1 / 2O2_>2H2+CO) may first proceed according to a partial combustion reaction CFk+O^Cb^CO+FFO and then according to a steam reforming reaction CH4+H2O^3H2+CO and / or a CO2 reforming reaction CH4+CO2^2H2+2CO.As one skilled in the art knows, the above general chemical reactions can be conducted in presence of nitrogen; in particular, adiabatic oxidations can be carried out with a gas containing oxygen and nitrogen. Nitrogen, being inert in reforming / oxidation reactions, is not contemplated in the above general reactions.The aforementioned process gases discharged at high temperature must be cooled in order to be subjected to subsequent unitary operations, such as water gas shift, CO2 removal and / or washing / purification. According to the present invention, the process gases downstream of the reforming / oxidation reactor can be subjected to:- a cooling with direct heat exchange with nitrogen, without the aid of process boilers and / or heat exchangers,- a cooling with direct heat exchange with nitrogen and a cooling with indirect heat exchange using process boilers and / or heat exchangers.The hot process gas discharged from the reforming / oxidation reactor, according to the present invention, is mixed and cooled with a colder nitrogen stream, preferably in cryogenic conditions, coming from a cryogenic air separation unit. Cold nitrogen can be mixed in liquid or vapor phase. The mixing lowers the temperature of the process gas; by regulating the temperature and / or flow rate of the nitrogen, it is possible to regulate the cooling of the process gas.It is emphasized that the direct nitrogen cooling operation disclosed by the present invention is distinct from conventional washing or purification operations carried out bymeans of cryogenic nitrogen in plants where process gas is synthesized for producing ammonia. These conventional cryogenic nitrogen washing / purification operations are performed downstream of water gas shift and carbon dioxide removal units, and are performed to remove undesired chemical species from the process gas. Consequently, according to these conventional operations, the cryogenic nitrogen is mixed with a process gas that has a very different chemical composition from the composition of the gas discharged from the reforming / oxidation reactor. Furthermore, according to these conventional washing / purification operations, the process gas mixed with the cryogenic nitrogen is already relatively cold and the removal of the undesired chemical species takes place since the mixing cryogenic nitrogen remains, at least partially, in liquid phase and / or the undesired chemical species condense.On the contrary, according to the invention here described, where the term “gas” is referred to both upstream and downstream of the mixing with nitrogen, the mixing between the high temperature process gas and the cold nitrogen takes place directly downstream of the reforming / oxidation reactor and does not involve any removal of chemical species and condensed streams from the process gas.If the cooling of the process gas downstream of the reforming / oxidation takes place with the aid of boilers and / or heat exchangers, the direct cooling with nitrogen can be carried out upstream or downstream of the boilers or heat exchangers, or in an intermediate position, i.e. between two exchangers.The unitary operation which the process gas is subjected to downstream of the nitrogen direct cooling can be of any type, for example a chemical conversion, a phase separation, a filtration, a rectification or distillation, an absorption or washing or purification.Downstream of the direct cooling with nitrogen, if the nitrogen concentration in the process gas is excessive for the subsequent chemical production, the excess nitrogen is separated and possibly recycled; the separation of nitrogen from the process gas can be carried out with any physical method, for example by adsorption on solid material (zeolites) or condensation / washing in cryogenic conditions.In the present description, an oxygen-containing stream or gas may correspond to atmospheric air, or enriched atmospheric air, i.e. air having a higher concentration ofoxygen than atmospheric, or pure oxygen, or a mixture of oxygen and other compounds, such as nitrogen.Following figures accompany the detailed description of the process gas synthesis method object of this invention:- Fig.l, where a simplified process diagram for performing the method of the invention in accordance with a first embodiment is shown;- Fig.2, where a simplified process diagram for performing the method of the invention in accordance with a second embodiment is shown;- Fig.3, where a simplified process diagram for performing the method of the invention in accordance with a third embodiment is shown;- Fig.4, where a simplified process diagram for performing the method of the invention in accordance with a fourth embodiment is shown;- Fig.5, where a simplified process diagram for performing the method of the invention for the subsequent production of ammonia in accordance with a first embodiment is shown;- Fig.6, where a simplified process diagram for performing the method of the invention for the subsequent production of ammonia in accordance with a second embodiment is shown;- Fig.7, where a simplified process diagram for performing the method of the invention for the subsequent production of ammonia in accordance with a third embodiment is shown;- Fig.8, where a simplified process diagram for performing the method of the invention for the subsequent production of ammonia in accordance with a fourth embodiment is shown;- Fig. 9 refers to diagram 1 showing operating parameters by injection of cold N2.In the process diagrams shown here, the lines or streams joining one or more units are indicated with arrows; in the description that follows, the terms “lines” or “streams” indicate both the fluid and the pipe or conduit or means apt for the fluid flow. The arrows indicate the flow direction of the fluid.In this description, the process gas pressure expressed in “MPa(a)” equals 106Pa absolute.Detailed description of the invention11SUBSTITUTE SHEET (RULE 26)Fig.1 shows the process diagram of the synthesis method of a process gas with direct cooling with nitrogen according to a first embodiment.The process diagram of Fig.1 comprises at least one stream of chemical reactants (1) to be introduced into a first unit (Ul) comprising at least one steam and / or CO2 reforming reactor or an adiabatic oxidation reactor with a stream containing oxygen. A first stream of reaction products, or a first process gas stream (3a), is extracted from the first unit (Ul), having a temperature higher than 500°C. The first stream (3a) has a pressure higher than 0,12MPa(a), or preferably higher than 0,20MPa(a), or more preferably higher than 0,45MPa(a).The process diagram of Fig.1 comprises an air separation unit (ASU) which an atmospheric air stream (4) is introduced in to obtain a cold nitrogen stream (5) and an oxygen stream (7). The cold nitrogen stream (5) is provided with a flow regulation system (6), such as a flowmeter and / or a valve. The cold nitrogen stream (5) is in fluid communication with the first process gas stream (3a) in a zone or mixing point (2) forming, by mixing, a second process gas stream (3b) constituted of the first process gas stream (3a) and the cold nitrogen stream (5); the second process gas stream (3b), therefore, has a higher nitrogen content and a lower temperature than the first process gas stream (3 a).The process diagram of Fig.1 comprises a second unit (U2) which the second process gas stream (3b) enters to be subjected to subsequent unitary operations, such as the water gas shift. Consequently, the first and second units (U1,U2) are connected to each other by the first and second process gas streams (3a, 3b) placed in series.The cold nitrogen stream (5) is preferably in cryogenic conditions; the cold nitrogen stream (5) can be in liquid or vapor phase. The cold nitrogen stream (5) has a pressure equivalent to or higher than the first process gas stream (3a) and has a lower temperature than the first process gas stream (3a). Preferably, the cold nitrogen stream (5) has a temperature below 0°C, preferably below -50°C, and more preferably below -100°C.It is emphasized that the second process gas stream (3b), after mixing with cold nitrogen (5), remains in gaseous conditions; this means that, if the cold nitrogen (5) is in liquid phase, the cold nitrogen is vaporized after mixing.Based on the embodiment of Fig.1, the method of the present invention substantially comprises following operations:- The separation of the air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5);- The introduction of at least one stream of chemical reactants (1) into the first unit (Ul), comprising hydrocarbons and / or ammonia and / or methanol;- The steam and / or CO2 reforming, or the adiabatic oxidation with an oxygencontaining stream, in the first unit (Ul) to synthesize the first process gas stream (3a) at high-temperature, rich in H2 / CO, or NOX, or HCN, or CH2O;- The cooling of the first stream of high temperature process gas (3a) by mixing with colder nitrogen (5) to obtain a cooled second process gas stream (3b) having a higher content of nitrogen;- The introduction of the second process gas stream (3b) into a second unit (U2) for carrying out subsequent unitary operations.Fig.2 shows the process diagram of the synthesis method of a process gas with direct cooling with nitrogen according to a second embodiment.The process diagram of Fig.2 is equivalent to that of Fig.1 except for the installation of a heat exchange unit (HE); in other words, the elements and the relative numbering of the process scheme shown in Fig.2 are equivalent to those of the process scheme shown in Fig. l. Therefore, for simplicity, the description of the process scheme of Fig.2 is partially omitted.Based on the embodiment of Fig.2, the heat exchange unit (HE) is installed between the first and second unit (Ul, U2) and, more specifically, it is installed downstream of the mixing point (2). The heat exchange unit (HE) is connected to the first unit (Ul) via the first and second process gas streams (3a, 3b), placed in series, and is connected to the second unit (U2) via a third process gas stream (3c). The heat exchange unit (HE) is suitable for the indirect heat exchange of the process fluid (3b) by the use of one or more auxiliary fluids. In other words, the heat exchange unit (HE) comprises one or more heat exchangers suitable for indirectly cooling the process gas (3b). Preferably, the heat exchange unit (HE) comprises a process boiler and, preferably, at least another heat exchanger. The heat exchange unit (HE) further cools the process gas to bring it to a temperature suitable for the unitary operations to be carried out in the second unit (U2). More specifically, the heat exchange unit (HE) cools the second process gas stream (3b) to obtain the third process gas stream (3c) at a lower temperature. The cold nitrogen stream (5) is connected to the first process gas stream (3a) at the mixing point (2).It is emphasized that the second process gas stream (3b), after mixing with cold nitrogen (5), and the third process gas stream (3c), after cooling in the heat exchange unit (HE), remain in gaseous conditions; this means that, if the cold nitrogen (5) is in liquid phase, the cold nitrogen is vaporized after mixing.Based on the embodiment of Fig.2, the method of the present invention substantially comprises following operations:- The separation of the air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5);- The introduction of at least one stream of chemical reactants (1) into the first unit (Ul), comprising hydrocarbons and / or ammonia and / or methanol;- The steam and / or CO2 reforming, or the adiabatic oxidation with an oxygencontaining stream, in the first unit (Ul) to synthesize the first process gas stream (3a) at high-temperature, rich in H2 / CO, or NOX, or HCN, or CH2O;- The cooling of the first process gas stream (3 a) by mixing with colder nitrogen (5) to obtain a cooled second process gas stream (3b) having a higher nitrogen content;- The introduction of the second process gas stream (3b) into a heat exchange unit (HE) for further cooling of the process gas by indirect heat exchange, so as to obtain a third process gas stream (3c) further cooled;- The introduction of the third process gas stream (3 c), exiting from the heat exchange unit (HE), into a second unit (U2) for carrying out subsequent unitary operation.Fig.3 shows the process diagram of the synthesis method of a process gas with direct nitrogen cooling according to a third embodiment.The process diagram of Fig.3 is equivalent to that of Fig.2 except for the location of the heat exchange unit (HE); in other words, the elements and the relative numbering of the process scheme shown in Fig.3 are equivalent to those of the process scheme shown in Fig.2. Therefore, for simplicity, the description of the process diagram of Fig.3 is partially omitted.Based on the embodiment of Fig.3, the heat exchange unit (HE) is installed between the first and second units (Ul, U2) and, more specifically, it is installed upstream of the mixing point (2). The heat exchange unit (HE) is connected to the first unit (Ul) via the first process gas stream (3a) and is connected to the second unit (U2) via the second and third process gas streams (3b ,3c), placed in series. The heat exchange unit (HE) receivesand cools the first process gas stream (3a), coming from the first unit (Ul), and then discharges the second process gas stream (3b) at a lower temperature. The heat exchange unit (HE) is suitable for the indirect heat exchange of the process fluid (3a) by the use of one or more auxiliary fluids. In other words, the heat exchange unit (HE) comprises one or more heat exchangers suitable for indirectly cooling the process gas (3a). Preferably, the heat exchange unit (HE) comprises a process boiler and, preferably, at least another heat exchanger. The cold nitrogen stream (5) is connected to the second process gas stream (3b), leaving the heat exchange unit (HE), at the mixing point (2).It is emphasized that the second process gas stream (3b), after cooling in the heat exchange unit (HE), and the third process gas stream (3c), after mixing with cold nitrogen (5), remain in gaseous condition; this means that, if the cold nitrogen (5) is in liquid phase, the cold nitrogen is vaporized after mixing.Based on the embodiment of Fig.3, the method of the present invention substantially comprises following operations:- The separation of the air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5);- The introduction of at least one stream of chemical reactants (1) into the first unit (Ul), comprising hydrocarbons and / or ammonia and / or methanol;- The steam and / or CO2 reforming, or the adiabatic oxidation with an oxygencontaining stream, in the first unit (Ul) to synthesize the first process gas stream (3a) at high-temperature, rich in H2 / CO, or NOX, or HCN, or CH2O;- The introduction of the first process gas stream (3a) (Ul) into the heat exchange unit (HE) for cooling the process gas by indirect heat exchange, so as to obtain a cooled second process gas stream (3b);- The cooling of the second process gas stream (3b) by mixing with colder nitrogen (5) to obtain a third process gas stream (3c) further cooled and having a higher content of nitrogen;- The introduction of the third process gas stream (3c) into a second unit (U2) for carrying out subsequent unitary operation.Fig.4 shows the process diagram of the synthesis method of a process gas with direct cooling with nitrogen according to a fourth embodiment.The process diagram of Fig.4 is equivalent to that of Fig.2 or Fig.3 except that there are two heat exchange units (HE1, HE2); in other words, the elements and the relative numbering of the process scheme shown in Fig.4 are equivalent to those of the process scheme shown in Fig.2 or Fig.3. Therefore, for simplicity, the description of the process diagram of Fig.4 is partially omitted.Based on the embodiment of Fig.4, the two heat exchange units (HE1, HE2) are installed between the first and second unit (Ul, U2); more specifically, the first heat exchange unit (HE1) is installed upstream of the mixing point (2) and the second heat exchange unit (HE2) is installed downstream of the mixing point (2). The first heat exchange unit (HE1) is connected to the first unit (Ul) via the first process gas stream (3a); the second heat exchange unit (HE2) is connected to the second unit (U2) via a fourth process gas stream (3d). The first heat exchange unit (HE1) is connected to the second heat exchange unit (HE2) via the second and third process gas streams (3b, 3c), placed in series. The cold nitrogen stream (5) is connected to the second process gas stream (3b), leaving the first heat exchange unit (HE1), at the mixing point (2). The first heat exchange unit (HE1) is suitable for the indirect heat exchange of the process fluid (3a) by the use of one or more auxiliary fluids. In other words, the first heat exchange unit (HE1) comprises one or more heat exchangers suitable for indirectly cooling the process gas (3a). Preferably, the first heat exchange unit (HE1) comprises a process boiler or a heat exchanger; according to another preferred embodiment, the first heat exchange unit (HE1) comprises a process boiler and at least another heat exchanger. The second heat exchange unit (HE2) is suitable for the indirect heat exchange of the process fluid (3c) by the use of one or more auxiliary fluids. In other words, the second heat exchange unit (HE2) comprises one or more heat exchangers suitable for indirectly cooling the process gas (3c).It is emphasized that the second process gas stream (3b), after cooling in the first heat exchange unit (HE1), the third process gas stream (3c), after mixing with cold nitrogen (5), and the fourth process gas stream (3d), after cooling in the second heat exchange unit (HE2), remain in gaseous conditions; this means that, if the cold nitrogen (5) is in liquid phase, the cold nitrogen is vaporized after mixing.Based on the embodiment of Fig.4, the method of the present invention substantially comprises following operations:- The separation of the air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and nitrogen (5);- The introduction of at least one stream of chemical reactants (1) into the first unit (Ul), comprising hydrocarbons and / or ammonia and / or methanol;- The steam and / or CO2 reforming, or the adiabatic oxidation with an oxygencontaining stream, in the first unit (Ul) to synthesize the first process gas stream (3a) at high-temperature, rich in H2 / CO, or NOX, or HCN, or CH2O;- The introduction of the first process gas stream (3a) (Ul) into a first heat exchange unit (HE1) for cooling the process gas by indirect heat exchange, so to obtain a cooled second process gas stream (3b);- The cooling of the second process gas stream (3b) by mixing with colder nitrogen (5) to obtain a third process gas stream (3c) further cooled and having a higher content of nitrogen;- The cooling of the third process gas stream (3c) obtained after mixing with cold nitrogen (5) by indirect heat exchange in a second heat exchange unit (HE2) to obtain a fourth process gas stream (3d) further cooled;- The introduction of the fourth process gas stream (3d) into the second unit (U2) for carrying out subsequent unitary operations.It should be emphasized that the embodiments shown in Figs.1-4, and related synthesis methods comprising the direct cooling with nitrogen, are applicable to any process gas belonging to those described above, such as process gases rich in H2 / CO for subsequent production of H2 / NH3 / CH3OH or Fisher Tropsch synthesis products, or rich in NOXfor subsequent production of HNO3, or rich in HCN for subsequent production of HCN, or rich in CH2O for subsequent production of CH2O.It should be noted that, for all the embodiments of Figs.1-4, the oxygen (7) coming from the air separation unit (ASU) can be introduced into the first unit (Ul) to carry out the adiabatic oxidation reactions; in other words, according to a preferential embodiment, the oxygen stream (7) can be connected to the first unit (Ul).As one skilled in the art can understand, the cold nitrogen stream (5) can be mixed with the process gas stream (3a, 3b, 3c, 3d) in several points; in other words, the synthesis method described here can foresee more than one mixing point (2). It should therefore be noted that cold nitrogen (5), on the basis of alternative and preferential embodiments of the present synthesis method, can be mixed with the hot process gas both upstream anddownstream of a heat exchange unit (HE, HE1, HE2), but always upstream of the second unit (U2).It should be also noted that the mixing point (2) can be inside the heat exchange unit (HE, HE1, HE2), i.e. the cold nitrogen (5) can be injected into a heat exchanger. Consequently, the direct nitrogen cooling can be an operation performed on a line or duct and / or in a heat exchanger.As one skilled in the art can understand, the cold nitrogen (5) and oxygen (7), produced in the air separation unit (ASU), can be stored in tanks or vessels; therefore, according to alternative embodiments of the present synthesis method, the cold nitrogen stream (5), or the cold nitrogen stream (5) and the oxygen stream (7), are connected to tanks or vessels instead of being directly connected to the air separation unit (ASU) as shown in Figs.1-4.Fig.5 shows the process diagram of the synthesis method of a process gas, with direct cooling with nitrogen, useful for the subsequent production of ammonia in accordance with a first embodiment of the present invention. More specifically, Fig.5 refers to a process scheme for producing ammonia by hydrocarbons adiabatic oxidation with oxygen, i.e. by means of an autothermal reformer or a partial oxidation or catalytic partial oxidation reactor (Ul).The general operational criteria related to the direct nitrogen cooling of the hot process gas described in Figs.1-4 also apply to the process diagram of Fig.5.The process diagram of Fig.5 includes at least:- a first unit (Ul) comprising at least one chemical reactor;- a heat exchange unit (HE) comprising at least one heat exchanger;- a second unit (U2) comprising at least one chemical reactor;- a third unit (U3) comprising at least one apparatus for purifying the process gas;- a fourth unit (U4) comprising at least one apparatus for purifying the process gas;- a fifth unit (U5) including equipment for the production of ammonia;- an air separation unit (ASU).The first unit (Ul) receives at least one stream of chemical reactants (1) and one stream of oxygen (7) coming from the air separation unit (ASU). The first unit (Ul) is connected to the heat exchange unit (HE) via a first and second process gas streams (3a, 3b) arranged in series, the heat exchange unit (HE) is connected to the second unit ( U2)via a third process gas stream (3c), the second unit (U2) is connected to the third unit (U3) via a fourth process gas stream (3d), the third unit (U3) is connected to the fourth unit (U4) by a fifth process gas stream (3e) and the fourth unit (U4) is connected to the fifth unit (U5) by a final synthesis gas stream (10). The air separation unit (ASU) is connected to the first process gas stream (3a) at a mixing point (2) located upstream of the heat exchange unit (HE) and, optionally, to the fourth unit (U4), respectively by means of a first and a second stream of cold nitrogen (5a, 5b). The first stream and the optional second stream of cold nitrogen (5a, 5b) are equipped with a flow regulation system, such as flowmeters and / or valves (6a, 6b).Based on the embodiment of Fig.5, the synthesis method of the present invention substantially comprises following operations:- The separation of the air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and cold nitrogen (5a, 5b);- The introduction of chemical reactants (1), including hydrocarbons, and oxygen (7) in the first unit (Ul);- The hydrocarbons adiabatic oxidation with oxygen (7), in the first unit (Ul), to synthesize a first process gas stream (3a) at high temperature and rich in H2 / CO;- The direct cooling of the first process gas stream (3a) leaving the first unit (Ul) by mixing with a first stream of cold nitrogen (5a), at the mixing point (2), to obtain a second process gas stream (3b) having a higher nitrogen content and a lower temperature than the first process gas stream (3a);- The cooling of the second process gas stream (3b) in the heat exchange unit (HE), by indirect heat exchange with at least one auxiliary fluid, to obtain a third process gas stream further cooled (3c);- The introduction of the third process gas stream (3c) into the second unit (U2) to carry out the water gas shift, according to the general reaction CO+H2O^CO2+H2, and obtain a fourth process gas stream (3d) lean in, or free from, CO and rich in CO2;- The introduction of the fourth process gas stream (3d) into the third unit (U3) to remove CO2 and / or acid chemical species (8) and to obtain a fifth process gas stream (3e);- The introduction of the fifth process gas stream (3e) and, possibly, of a second stream of cold nitrogen (5b), preferably liquid, in the fourth unit (U4) to removechemical species in excess and / or oxygenated chemical species and / or impurities (9) and obtain a final synthesis gas stream (10) consisting essentially of hydrogen and nitrogen, with a hydrogen to nitrogen molar ratio of about 3: 1;- The introduction of the final synthesis gas stream (10) into the fifth unit (U5) to produce ammonia (11), according to the general reaction 3H2+N2^2NH3.Fig.6 shows the process diagram of the synthesis method of a process gas, with direct cooling with nitrogen, useful for the subsequent production of ammonia according to a second embodiment. More specifically, Fig.6 refers to a process scheme for producing ammonia by hydrocarbons adiabatic oxidation with an oxygen / nitrogen mixture, i.e. by an autothermal reformer or a partial oxidation or catalytic partial oxidation reactor (Ul).The process diagram of Fig.6 is equivalent to that of Fig.5 except for the installation of a third stream of nitrogen (5c); in other words, the elements and the relative numbering of the process scheme shown in Fig.6 are equivalent to those of the process scheme shown in Fig.5. Furthermore, the operations or main units of Fig.6 are equivalent to those of Fig.5. Therefore, for simplicity, the description of the process scheme of Fig.6 and the relative method are partially omitted.The general operational criteria related to the direct nitrogen cooling of the hot process gas described in Figs.1-4 also apply to the process diagram of Fig.6.According to the embodiment of Fig.6, a third nitrogen stream (5c), obtained from the air separation unit (ASU), is connected to the oxygen stream (7); the third nitrogen stream (5c) is provided with a third flow regulation system (6c). The third nitrogen stream (5c) is mixed with the oxygen stream (7) to obtain an oxygen / nitrogen mixture (12). The oxygen / nitrogen mixture (12) is introduced into the first unit (Ul) to carry out the adiabatic oxidation of the hydrocarbons. The possibility of mixing oxygen and nitrogen allows regulating the temperature and yield of the adiabatic oxidation reaction. As one skilled in the art can understand, the total amount of nitrogen present in the final synthesis gas stream (10) is obtained by adjusting the amount of nitrogen related to the first, second (if present) and third streams (5a, 5b, 5c). It should be noted that the nitrogen relating to the third stream (5 c) preferably has a higher temperature than that of the first and second stream of nitrogen (5a, 5b) to promote the oxidation reaction. Preferably, the temperature of the oxygen / nitrogen stream (12) is higher than 150°C and, preferably, higher than 250°C.Fig.7 shows the process diagram of the synthesis method of a process gas, with direct cooling with nitrogen, useful for the subsequent production of ammonia according to a third embodiment. More specifically, Fig.7 refers to a process scheme for producing ammonia by hydrocarbons adiabatic oxidation with air, i.e. by means of an autothermal reforming or secondary reforming reactor (Ul).The general operational criteria related to the direct nitrogen cooling of the hot process gas described in Figs.1-4 also apply to the process diagram of Fig.7.The process diagram of Fig.7 is equivalent to that of Fig.5 except for the injection of air (4b) instead of oxygen in the first unit (Ul) and for the installation of a sixth unit (U6); in other words, the elements and the relative numbering of the process scheme shown in Fig.7 are equivalent to those of the process scheme shown in Fig.5. Furthermore, the operations or main units of Fig.7 are equivalent to those of Fig.5. Therefore, for simplicity, the description of the process diagram of Fig.7 and the relative method are partially omitted.Based on the embodiment of Fig.7, the synthesis method of the present invention substantially comprises the same operations relating to Fig.5 except for the operation of introducing air (4b) into the first unit (Ul), to carry out the adiabatic oxidation of hydrocarbons, and the methanation operation in the sixth unit (U6), installed between the third and fourth units (U3, U4). Consequently, a first air stream (4a) is introduced into the air separation unit (ASU) while a second air stream (4b) is introduced into the first unit (Ul). The sixth unit (U6) receives the fifth process gas stream (3e), leaving the third unit (U3); in the sixth unit (U6) the methanation operation is carried out, according to the general reactions CO+OFF^CIU+IUO and CO2+4H2^CH4+2H2O, to completely or almost completely eliminate carbon oxides from the process gas, forming methane and water, in the presence of a solid catalyst. A sixth process gas stream (3f) exits from the sixth unit (U6) and is introduced into the fourth unit (U4) for the purification of the process gas and for the possible injection of a second stream of cold nitrogen, (5b), preferably liquid, to be put in direct contact with the process gas (3f).As one skilled in the art can understand, the embodiment of Fig.7 can be easily modified by injecting the oxygen stream (7) into the second air stream (4b) to obtain air enriched in oxygen to be injected into the first unit (Ul); consequently, the first unit (Ul) performs adiabatic oxidation of hydrocarbons by means of enriched air.Fig.8 shows the process scheme of the synthesis method of a process gas, with direct cooling with nitrogen, useful for the subsequent production of ammonia according to a fourth embodiment. More specifically, Fig.8 refers to a process diagram for producing ammonia by hydrocarbons steam and / or CO2 reforming.The general operational criteria related to the direct nitrogen cooling of the hot process gas described in Figs.1-4 also apply to the process diagram of Fig.8.The process diagram of Fig.8 includes at least:- a first unit (Ul) comprising at least one chemical reactor;- a heat exchange unit (HE) comprising at least one heat exchanger;- a second unit (U2) comprising at least one chemical reactor;- a third unit (U3) comprising at least one apparatus for purifying the process gas;- a fourth unit (U5) including equipment for the production of ammonia;- An air separation unit (ASU).The first unit (Ul) receives at least one stream of chemical reactants (1). The first unit (Ul) is connected to the heat exchange unit (HE) by means of a first and second process gas streams (3a, 3b) placed in series, the heat exchange unit (HE) is connected to the second unit ( U2) via a third process gas stream (3c), the second unit (U2) is connected to the third unit (U3) via a fourth process gas stream (3d), the third unit (U3) is connected to the fourth unit (U5) via a fifth process gas stream (3e) and a final synthesis gas stream (10) placed in series. The air separation unit (ASU) is connected to the first process gas stream (3 a) and, optionally, to the fifth process gas stream (3e) respectively via a first and via an optional second cold nitrogen stream (5a, 5b ), and respectively at a first and at a second mixing point (2a, 2b). The first stream and the optional second stream of cold nitrogen (5a, 5b) are equipped with a flow regulation system, such as flowmeters and / or valves (6a, 6b).Based on the embodiment of Fig.8, the synthesis method of the present invention substantially comprises following operations:- The separation of the air (4) by means of the separation unit (ASU) to obtain streams of oxygen (7) and cold nitrogen (5a, 5b);- The introduction of chemical reactants (1), including hydrocarbons, steam and / or CO2, into the first unit (Ul);- The hydrocarbons steam and / or CO2 reforming, in the first unit (Ul), to synthesize a first process gas stream (3a) at high temperature and rich in H2 / CO;- The cooling of the first process gas stream (3a) leaving the first unit (Ul) by mixing with a first stream of cold nitrogen (5a), at a first mixing point (2a), to obtain a second gas process stream (3b) having a higher nitrogen content and a lower temperature than the first process gas stream (3a);- The cooling of the second process gas stream (3b) in the heat exchange unit (HE), by indirect heat exchange with at least one auxiliary fluid, to obtain a further cooled third process gas stream (3c);- The injection of the third process gas stream (3c) into the second unit (U2) to carry out the water gas shift and obtain a fourth process gas stream of (3d) lean in, o free from, CO and rich in CO2;- The introduction of the fourth process gas stream (3d) into the third unit (U3) to remove CO2, acid chemical species, impurities and chemical species in excess (8) and to obtain a fifth process gas stream (3e) essentially constituted of hydrogen or hydrogen and nitrogen;- The optional mixing of the fifth process gas stream (3e) with a second stream of nitrogen (5b), at the second mixing point (2b), to obtain a stream of final synthesis gas (10) essentially constituted of hydrogen and nitrogen, with a hydrogen to nitrogen molar ratio of about 3: 1;- The introduction of the final synthesis gas stream (10) into the fourth unit (U5) to synthesize ammonia (11).It should be noted that, for the embodiment of Fig.8, the nitrogen relating to the second stream (5b), if present, has preferably a higher temperature than that of the first nitrogen stream (5a); it should be emphasized that the second nitrogen stream (5b), if present, has preferably the scope of adjusting the composition of the final synthesis gas (10) rather than cooling the process gas (3e).For the embodiments of Figs.5-8, the second nitrogen stream (5b) is shown by a dotted line; in fact, it is emphasized that the second nitrogen stream (5b) is optional. The second nitrogen stream (5b) relating to Figs.5-8 is useful for adjusting the composition of the final synthesis gas (10) and / or for carrying out a final purification of the process gas in cryogenic conditions. In the latter case, the second nitrogen stream (5b) is in cryogenic conditions and, preferably, in liquid state so as to carry out a washing of the process gasand eliminate undesired residual chemical species (for example methane, water, carbon oxides, incondensable gases). As per above, an expert in the field can easily foresee that, if the second stream of cold nitrogen (5b) is present, the flow rate of the first nitrogen stream for the direct cooling (5a), injected at the mixing point (2) upstream of the second unit (U2), and the flow rate of the second nitrogen stream (5b), injected downstream of the second unit (U2), can be regulated so that the synthesis operations are optimized; in other words, the expert in the field can easily foresee that the total amount of nitrogen present in the final synthesis gas stream (10) can be obtained by regulating the amount of nitrogen relative to the first and second streams (5a, 5b).In accordance with an alternative embodiment of the method object of the present invention and related to the embodiments of Figs.5-8, the hot process gas discharged from the first unit (Ul) is cooled by mixing with cold nitrogen downstream of the heat exchange unit (HE) and upstream of the second unit (U2); in other words, the mixing point (2) is located downstream of the heat exchange unit (HE). This alternative embodiment relating to Figs.5-8 complies with the general operating criteria described for the embodiments relating to Figs.1-4.In accordance with an alternative embodiment of the method object of the present invention and relating to the embodiments of Figs.5-8, the hot process gas discharged from the first unit (Ul) is cooled by several mixings with cold nitrogen; in other words, the hot process gas is mixed with the cold nitrogen through multiple and distinct mixing points positioned between the first and second unit (Ul, U2). This alternative embodiment relating to Figs.5-8 complies with the general operating criteria described for the embodiments relating to Figs.1-4.It should be noted that, for all the embodiments relating to Figs.1 -8, the first unit (Ul) where the reforming / oxidation operation takes place can comprise more than one reforming / oxidation reactor. For example, as one skilled in the art knows, for the production of hydrogen, ammonia, methanol or Fischer Tropsch synthesis products, a steam / CCh reformer and an adiabatic oxidation reactor with oxygen-containing gas are sometimes used and arranged in series or in parallel.As one skilled in the art can understand, for all embodiments of the synthesis method described herein, additional equipment and / or units can be inserted between one unit andanother, such as heat exchangers, phase separators, compressors or pumps, washing or rectification columns, without modifying the inventive concept of the present invention.As one skilled in the art can understand, the units (Ul, U2, U3, U4, U5, U6, HE, HE1, HE2, ASU) described in the embodiments of the process gas synthesis method may include various equipment, such as tanks, heat exchangers, rectification columns, phase separators, rotating machines, without modifying the inventive concept of the present invention.It is emphasized that the synthesis method disclosed by this invention has the scope to cool the process gas; consequently, the flow rate and / or the temperature of the cold nitrogen stream injected into the hot process gas stream must be adequate for cooling. Preferably, the flow rate of the nitrogen stream corresponds to at least 2% by mass of the hot process gas flow rate; more preferably, the flow rate of the nitrogen stream corresponds to at least 5% by mass of the hot process gas flow rate. The cold nitrogen is preferably in cryogenic conditions; the temperature of the cold nitrogen is preferably below 0°C, preferably below -50°C and, more preferably, below -100°C. The hot process gas temperature, after mixing with cold nitrogen, is lowered preferably by at least 10°C, preferably by at least 25°C and, more preferably, by at least 50°C.The cooling operation of the process gas, discharged at high temperature from the reforming / oxidation reactor, by the direct contact with cold nitrogen, and performed upstream of any further unitary operation, has following features:- the cooling of process gas is rapid and takes place with a limited pressure drop;- Nitrogen is an inert chemical species, therefore it does not substantially modify the synthesis chemistry and does not attack construction steels;- The cooling nitrogen can be subsequently removed from the process gas by a physical separation method (such in units U3 and / or U4);- If the process gas is used for the subsequent production of ammonia, the cooling nitrogen is used as a chemical reagent in the ammonia production unit (U5);- The process gas cooled by mixing with nitrogen remains in gaseous condition.In accordance with the above detailed description, it is therefore clear that this synthesis method allows obtaining the advantages described above and allows solving some potential operational problems. In particular, for the existing production plants, the method object of the present invention offers following specific advantages:- In case of temperature peaks or uncontrolled reactions in the first unit (Ul), where the process gas is synthesized at high temperature, the direct cooling with nitrogen allows mitigating the temperature of the process gas entering the next unit (HE, U2) and therefore allows protecting it from possible overheating and / or damage. In this case, the method object of the present invention has an emergency or safeguard cooling function;- The thermo-hydraulic and thermo-mechanical conditions of the equipment included in the heat exchange unit (HE), i.e. the heat exchangers, are mitigated, and therefore the equipment work under less severe conditions and their operational life can be extended. In this case, the method object of the present invention can be used for a modernization and / or an upgrading of the existing plant;- If the heat exchange unit (HE) includes process boilers, the steam production can be decreased or optimised. In this case, the method object of the present invention can be used for a modernization of the existing plant;- If a heat exchanger apt to cool the process gas by means of indirect heat exchange suffers from failure or performance decrease, the direct cooling with nitrogen allows partially or totally compensating for the limited heat exchange by the heat exchanger. In this case, the method object of the present invention has an emergency or temporary cooling function.The method for synthesizing a process gas by direct cooling with nitrogen, as conceived and described, is in any case subject to numerous modifications and variations, all attributable to the same inventive concept. Furthermore, all the details may be replaced with technically equivalent elements.The protective scope of the present invention is defined by the attached claims.ExampleTypical chemical composition and operating conditions of a process gas discharged from an autothermal reforming reactor, i.e. from a reactor for the adiabatic oxidation with oxygen of a hydrocarbon feedstock in the presence of a solid catalyst and for subsequent ammonia production, are reported here below:In accordance with a conventional synthesis method, the process gas is conventionally cooled, downstream of the adiabatic oxidation reactor, by at least one shell-and-tube process gas boiler (PGB), with process gas flowing on tube-side and boiling water flowing on shell-side typically at saturation temperature of 320°C. After conventional cooling, the process gas undergoes a unitary operation, such as the water gas shift.The conventional synthesis method is modified according to the present invention; more specifically, the present example consists of inserting a mixing point between the oxidation reactor and the PGB to directly cool the hot process gas with cold nitrogen.Nitrogen available for direct cooling of the process gas is at a temperature of -140°C and a pressure of 3.0MPa(a); the cooling nitrogen is in gas phase and is mixed with the process gas upstream of the PGB.Assuming a flow rate of the hot process gas leaving the reactor of lOOOOkg / h and assuming that mixing is carried out under ideal thermodynamic and fluid dynamic conditions, direct cooling with nitrogen allows to approximately obtain No. 3 operating parameters shown in Diagram 1 of Fig. 9.Specifically, the Diagram 1 shows the profile of steam production in the PGB (“Steam prod, in PGB”), the temperature of the process gas at the inlet of the PGB (“Inlet gas temp, in PGB”) and the heat flux at the inlet of the PGB exchanging tubes (“Inlet heat flux in PGB”) against the change of the flow rate of cold nitrogen (on the abscissa) injected in the hot process gas leaving the reactor.27SUBSTITUTE SHEET (RULE 26)The three operating parameters are at 100% without injection and direct cooling with nitrogen, i.e. in the case of conventional synthesis method. As the Diagram 1 shows, according to the synthesis method object of the present invention, the direct cooling with nitrogen allows decreasing the three operating parameters. For example, by injecting and mixing a flow rate of cold nitrogen equal to 25% (2500kg / h) of the hot process gas flow rate, it is possible to reduce the heat flux and the gas temperature at the PGB inlet by more than 10% and to reduce steam production in the PGB by almost 10%. Thermo-mechanical and thermo-hydraulic operating conditions of the PGB are lessened as:- the metal temperatures of the exchanging tubes at the PGB inlet decrease significantly,- the vapor fraction in the PGB decreases and the two-phase water cooling significantly improves.A person skilled in the art can understand that the numerical example described herein is, for example, convenient in case the PGB has a portion of exchanging tubes that are plugged. In this case, at the nominal plant load, the PGB works in more critical thermo-hydraulic and thermo-mechanical conditions since the total flow rate of the hot process gas passes through only a portion of the exchanging tubes. By using the synthesis method object of the present invention, i.e. by performing a direct cooling with nitrogen upstream of the PGB, it is possible to mitigate the operating conditions of the PGB and continue to operate the plant without the risk of further damage to the PGB tubes.28SUBSTITUTE SHEET (RULE 26)

Claims

Claims1. Synthesis method of a process gas, for the subsequent production of at least one of the chemicals among hydrogen, ammonia, methanol, olefins / paraffins / alcohols from Fischer Tropsch synthesis, nitric acid, hydrogen cyanide, formaldehyde, comprising following operations:The injection of at least one chemical reactants stream containing hydrocarbons and / or ammonia and / or methanol into a first unit (Ul);The steam and / or CO2 reforming, or the adiabatic oxidation with oxygen-containing gas, of said hydrocarbons and / or ammonia and / or methanol for synthesizing the process gas at high temperature in the first unit (Ul);The cooling of the process gas directly downstream of said reforming or oxidation;The injection of the cooled process gas in a second unit (U2) for further unitary operations; said method being characterized in that said cooling comprises at least a mixing operation of the process gas with a colder stream constituted of, or essentially constituted of, nitrogen.

2. Method according to claim 1, wherein the process gas synthesized in said first unit (Ul) contains hydrogen (H2) and carbon monoxide (CO) respectively having a molar concentration of at least 20% and 3%, or nitrogen oxides (NOX) having a total molar concentration of at least 3%, or hydrogen cyanide (HCN) having a molar concentration of at least 3%, or formaldehyde (CH2O) having a molar concentration of at least 3%.

3. Method according to claim 1 or 2, wherein said reforming or adiabatic oxidation proceeds according to at least one of following general chemical reactions:- CnHm+nH2O^(n+m / 2)H2+nCO ;- CH4+CO2^2H2+2CO ;- CnHm+n / 2O2^m / 2H2+nCO ;- 4NH3+5O2->6H2O+4NO ;- CH4+NH3+3 / 2O2^3H2O+HCN ;- CH3OH+1 / 2O2^H2O+CH2O .

4. Method according to claim 2 or 3, wherein the process gas synthesized in said first unit (Ul) has a temperature equal to or greater than 500°C.

5. Method according to claim 2 or 3, wherein the process gas synthesized in said first unit (Ul) has a pressure equal to or greater than 250000Pa absolute.

6. Method according to anyone of claims from 1 to 5, wherein said cooling comprises also an operation of indirect heat exchange through at least one heat exchanger.

7. Method according to claim 6, wherein said at least one heat exchanger is a process gas boiler cooling the process gas by means of pressurized boiling water.

8. Method according to anyone of claims from 1 to 7, wherein said nitrogen stream is produced in an air separation unit (ASU).

9. Method according to claim 4 or 5, wherein in said second unit (U2) the process gas is subjected to the water gas shift reaction for converting carbon monoxide (CO) into carbon dioxide (CO2).

10. Method according to claim 9, comprising downstream of said second unit (U2) following further operations:The removal of carbon monoxide, carbon dioxide, water, and hydrocarbons from the process gas;The preparation of a mixture constituted of, or essentially constituted of, hydrogen and nitrogen with a molar ratio of 3 : 1 approx.;The production of ammonia.

11. Method according to claim 1, wherein said oxygen-containing gas is atmospheric air, or oxygen-enriched atmospheric air, or an oxygen / nitrogen.

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